1998 Volkswagen Polo Engine misfireing

Since driving through a large puddle my endinge has been misfiring. This only seems to happen when the engine is under pressure. I have replaces the spark plugs and the distrutor cap.
Nothingin the engine seems to be damp still from the wetness.
Also when it rains the engine takes a while to start and then misfires untill it is warmed up